1. Field of the Invention
This invention relates to lead-acid batteries and, more particularly, to a bipolar lead-acid battery.
2. Description of the Prior Art
Lead-acid batteries and cells have been known for a substantially long period of time and have been employed commercially in a relatively wide variety of applications. Such applications have ranged from starting, lighting and ignition for automobiles, trucks and other vehicles (often termed "SLI batteries") to marine and golf cart applications and to various stationary and motive power source applications (sometimes termed "industrial battery applications").
The lead-acid electrochemical system has provided a reliable energy source, and the resulting batteries are amenable to automated production with a high quality standard. However, one serious drawback of either the flooded or sealed, absorbed electrolyte lead-acid batteries is their relatively low energy and power density. It has long been a desire to provide an energy source with the reliability of a flooded or sealed lead-acid battery system while at the same time achieving much higher energy and power densities.
Thus, as one example, a true bipolar battery (i.e., the positive and negative plates in some fashion share the same conductive grid or substrate) is capable of providing energy performances at 20 hour rates of about 35-65 watt-hours/kg. and 90-160 watt-hours/liter in comparison to 35-47 watt-hours/kg. and about 50-66 watt-hours/liter for what has been termed a quasi-bipolar battery (i.e., while not sharing the same grid or substrate, the positive and negative plates are connected by multiple connections such as shown in U.S. Pat. No. 4,209,575 to McDowall et al.). As regards the power density capability, a true bipolar battery should be capable providing about 1.3 to 6.0 kilowatts/kg. and 3.2 to 14 kilowatts/liter in comparison to about 0.9 kilowatts/kg. and 1.2 kilowatts/liter for a quasi-bipolar battery. The comparative difference in the power and energy density capabilities between a true bipolar and a conventional lead-acid battery design will be even more dramatic. In addition, the inherent uniform current distribution characteristic of a bipolar lead-acid battery in comparison to that exhibited by a conventional lead-acid battery should result in an overall increase in the active material utilization and battery cycle life.
For these reasons, considerable effort over the last 20 years has been directed to developing lead-acid and other electrochemical systems in a bipolar design. U.S. Pat. No. 3,728,158 to Poe et al. discloses a low silhouette, bipolar electrode battery stack in which several cells are individually vented along the side of the battery to a venting manifold. U.S. Pat. No. 4,125,680 to Shropshire et al. discloses a plurality of bipolar carbon-plastic electrode structures that are formed by first molding thin conductive carbon-plastic sheets from heated mixtures of specified carbon and plastic and then establishing frames of dielectric plastic material around the sheets and sealing the frames to the sheets so as to render the resulting structures liquid impermeable.
U.S. Pat. No. 4,964,878 to Morris discloses a method of making a recombinant lead-acid battery which comprises assembling stacks of plates in such a manner that a positive plate in a particular position in one stack is connected to a negative plate in the same relative position in an adjacent stack by a common substrate of the positive and negative plates. U.S. Pat. No. 5,068,160 to Clough et al. discloses an assembly of plates, spacer members and thermoplastic polymer frame elements which are bonded together.
Still further, U.S. Pat. No. 4,542,082 to Rowlette discloses a variety of approaches for providing bipolar plates, More particularly, it is noted that most batteries utilizing bipolar plates have utilized metal substrates such as lead or lead alloys. After setting forth the problems with such an approach, Rowlette states that a different approach must be used if acceptable battery weight and service life are to be simultaneously achieved. Alternative approaches, Rowlette identifies, have included plates formed by dispersing conductive particles or filaments such as carbon, graphite, or metal in a resin binder such as polystyrene incorporating therein metal or graphite powder (U.S. Pat. No. 3,202,545), a plastic frame of polyvinylchloride with openings carrying a battery active paste mixed with nonconductive fibers and short non-contacting lead fibers for strengthening the substrate (U.S. Pat. No. 3,466,193), a bi-plate having a layer of zinc and a polyisobutylene mixed with acetylene black and graphite particles for conductivity of the plate (U.S. Pat. No. 3,565,694), a substrate for a bipolar plate including polymeric material and vermicular expanded graphite (U.S. Pat. No. 3,573,122), a rigid polymer frame having a grid entirely of lead filled with battery paste (U.S. Pat. No. 3,738,871), a plastic thin substrate having lead stripes on opposite faces, the lead stripes being interconnected through an opening in the substrate and retained by plastic retention strips (U.S. Pat. No. 3,891,412), and a bi-plate having a substrate of thermoplastic material filled with finely divided vitreous carbon and a layer of lead antimony foil bonded to the substrate for adhering active materials (U. S. 4,098,967).
Rowlette further references U.S. Pat. No. 4,275,130 in which the bipolar plate construction comprises a thin composite of randomly oriented conductive graphite, carbon or metal fibers embedded in a resin matrix with stripes of lead plated surfaces thereof. Still further reference is made to Rowlette's then-pending application which includes a bi-plate formed of a thin sheet of titanium covered with a layer of epoxy resin containing graphite powder.
In the '082 Rowlette patent, the bipolar plate is described as being formed of a continuous sheet of a resinous material containing a plurality of spaced conductors extending from a first surface to the second surface thereof. The conductors are sealingly received in the sheet of resin in such a fashion that no liquid passes between the resin enveloping the end of the conductor facing each surface thereof.
Still further examples of bipolar electrochemical plates are set forth in U.S. Pat. Nos. 4,637,970 and 4,683,648 to Yeh et al. The bipolar electrodes described comprise a core portion composed of titanium and an integral, substantially continuous and non-porous layer of lead electroplated onto at least one surface of the core portion and diffused a selected distance into the core portion.
Yet, despite the substantial advantages that could be achieved using bipolar batteries and cells and the substantial amount of work and attention directed to this type of battery over at least the last 20 years, it seems that bipolar lead-acid batteries have remained largely a very promising laboratory curiosity. At least the vast majority of the applications where a bipolar lead-acid battery would be most advantageous (e.g., SLI, electric vehicle and hybrid electric vehicle) require capacities that cannot be readily obtained because of the size of the plates that would be required. It is thus quite difficult to provide a bipolar design that will have the desired capacity but will also meet the limited space requirements. Providing a conductive metal substrate that can satisfy the strength and corrosion resistance requirements has also been, it seems, an insurmountable problem. Achieving satisfactory paste adhesion and venting have also proven to be difficult tasks. Reliable electrolyte-free sealing means between adjacent bi-plates and cells has proved to be difficult in the past and has been one of the problems. Thus, there still exists the need for a bipolar battery which will achieve the enhanced electrochemical performance that a bipolar battery design can provide while satisfactorily dealing with the diverse problems identified by the prior art in a reliable manner.
It is accordingly a principal object of the present invention to provide a reliable but practical bipolar lead-acid battery.
Another object of the present invention is to provide a bipolar battery modular in design which allows the ease of increase in capacity without the necessity of increasing the size of the plates.
A further object of this invention lies in the provision of a bipolar battery characterized by high active material utilization and improved cycle life.
A still further object provides a bipolar battery employing a conductive metal substrate.
Another object of the present invention lies in the provision of a bipolar battery having enhanced paste adhesion characteristics.
Yet another object provides a bipolar battery having desirable venting capabilities.
These and other objects and advantages of the present invention will be apparent from the following description and drawings.